Part 10 (2/2)

[Ill.u.s.tration: FIG. 32.--Diagram to explain origin of regions of defective intensity.]

If this be the correct explanation, the path of the originating fault may be taken as that indicated by the broken line in Fig. 28, a line which is nearly parallel to the chief branches of the isoseismal curves.[46] As both epicentres lie on the west side of this line, the fault must hade or slope in this direction. The distortion of the Woodstock isoseismals towards the north-west confirms the latter inference, for the intensity of the shock is greater on the side towards which the fault hades.

From the comparative absence of earthquakes in South Carolina, we may infer that the fault is one subject to displacements at wide intervals of time. The gradually increasing stress along its surface was relieved at one or two points in or near the Woodstock focus on August 27th and 28th, and perhaps during the preceding months. But the first great slip took place suddenly in that focus, and spread gradually southwards--for there was no interruption in the movement--until about half-a-minute later it reached the Rantowles focus, where the second great slip occurred. Eight or ten minutes afterwards there was another slip--in what part of the fault is uncertain--and this was followed at irregular intervals by many small movements gradually diminis.h.i.+ng in frequency and in focal area. Within a year from the first disturbance, the fault-system attained once more its usual condition of rest.

REFERENCES.

1. DUTTON, C.E.--”The Charleston Earthquake of August 31st, 1886.”

_Amer. Geol. Survey, Ninth Annual Report_, pp. 209-528.

2. _Nature_, vol. x.x.xv., 1887, pp. 31-33; vol. lxiii., 1901, pp.

165-166.

FOOTNOTES:

[38] The authorities for this statement are Mallet's Catalogue of Recorded Earthquakes (_Brit. a.s.soc. Rep._, 1852, pp. 1-176; 1853, pp.

117-212; 1854, pp. 1-326), which closes with the year 1842, and Fuchs'

_Statistik der Erdbeben von 1865-1885_. According to Mallet, there was an earthquake in S. Carolina in November 1776, and the New Madrid earthquake of December 16th, 1811, was felt at Charleston. Fuchs records two earthquakes at Charleston on May 12th, 1870, and December 12th, 1876; and two in S. Carolina on December 12th and 13th, 1879.

[39] 1. Recorded by a single seismograph, or by some seismographs of the same pattern, but not by several seismographs of different kinds, the shock felt by an experienced observer.

2. Recorded by seismographs of different kinds; felt by a small number of persons at rest.

3. Felt by several persons at rest; strong enough for the duration or direction to be appreciable.

4. Felt by several persons in motion; disturbance of movable objects, doors, windows; creaking of floors.

5. Felt generally by every one; disturbance of furniture and beds; ringing of some bells.

6. General awaking of those asleep; general ringing of bells; oscillation of chandeliers, stopping of clocks; visible disturbance of trees and shrubs; some startled persons leave their dwellings.

7. Overthrow of movable objects, fall of plaster, ringing of church bells, general panic, without damage to buildings.

8. Fall of chimneys, cracks in the walls of buildings.

9. Partial or total destruction of some buildings.

10. Great disasters, ruins, disturbance of strata, fissures in the earth's crust, rock-falls from mountains.

[40] In order to simplify these figures, the rivers, most of the inlets, and other details are omitted. Small figures are added along the railway lines to denote the distance in miles from the stations in Charleston.

[41] If this were so, the decrease in intensity would be only apparent; but it may have been real, and a possible explanation on this supposition is given later on (p. 135).

[42] If _c_ be the depth of the focus, _a_ the intensity at unit distance from the focus, and _y_ the intensity on the surface at distance _x_ from the epicentre, then

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